1. Field of the Invention
The present invention relates to a method for treating the interior surfaces of microscopic penetrating holes provided on a workpiece and, more particularly, to treatment of through holes of a printed circuit board.
2. Description of the Related Art
Recently, as electronic instruments have been developed to further improve operation speed and calculation capacity, printed circuit boards used for system boards, memory boards or the like have been increasingly required to have accordingly higher circuit density. Owing to such a density increase each printed circuit board is laminated to have the layers further multiplied and thickened, and the through holes for electrically connecting these layers with one another must be reduced in diameter. As a result, it is difficult to realize the requisite condition by subjecting the interior surface of each small-diameter hole to chemical treatment such as plating. Especially in plating of the through holes, plating precipitation is interfered with by residual air in the through holes when dipping the workpiece into treating liquid and by gas and reaction products generated and remaining within the through holes during the treatment. This results in such problems as plating voids and decreases of plating thicknesses at the intermediate areas of the holes i.e., an insufficent plating condition of the inner surface of the through holes. Conventionally, in order to solve these problems, there have been known such methods as disclosed in Japanese Examined Patent Publication No. 61-13399, Japanese Unexamined Patent Publication No. 61-163695, Japanese Examined Patent Publication No. 61-46559, Japanese Examined Utility Model Publication No. 62-1240, and Japanese Examined Patent Publication Nos. 62-29518, 62-34837, and 62-37840.
Among these publications, the method disclosed in Japanese Examined Patent Publication No. 61-13399 is known for facilitating uniform plating of the interior surfaces of extremely thin through holes not only by controlling flows of plating liquid toward these through holes but also by regulating a distance from the electrode according to measurements of precipitation voltage and current.
In addition to this method, the method disclosed in Japanese Unexamined Patent Publication No. 61-163695 is known for supplying treating liquid directly into through holes of a printed circuit board and circulating the treating liquid through them.
In these conventional methods, however, in spite of the direct supply of liquid flows into the through holes the smallness of the hole diameter has brought about the difficulty of allowing the liquid to flow at sufficient speed, and furthermore, residual air within the through holes and remaining gas and reaction products generated during the treatment have been hard to thoroughly remove.
Besides, in this condition, the interior surfaces of the through holes have not been treated uniformly, and decreases of plating thicknesses at the intermediate areas of the holes have not been completely prevented.
Furthermore, among the above publications, in the method disclosed in Japanese Examined Patent Publication No. 61-46559, multi-fins are provided at both sides of a board to be treated, and repeatedly moved upwardly and downwardly at regular intervals to uniformly stir treating liquid, but such stirring only causes random liquid flows, which are not adequate for urging regular replacement of the treating liquid in through holes.
In another method, the treating liquid is directly delivered into the holes of the board to be treated, but it is difficult to apply this method to high-density printed circuit boards each including thousands to tens of thousands of through holes.
Further, in a different method, pressure is exerted to the treating liquid for delivering it from one surface to the other surface of the board to be treated. This method has such problems that the apparatus becomes larger and more complicated, and that when the board in question includes holes of different diameters, holes of smaller diameters are difficult for the treating liquid to flow through.
Now, the problems will be concretely explained with reference to the attached drawings. As shown in FIG. 5, a laminated board 1 having a thickness of 7 mm, through which holes 2 having a diameter of 0.3 mm penetrate, is held within a board cage 10, and dipped in thick-covering chemical copper plating liquid 12 available from the market for subjecting the holes to copper plating treatment in a plating tank 11. After the plating treatment with a desired value of a plating thickness being 30 .mu.m, when several sample pieces are cut out of the laminated board 1 for sectionally observing the thin holes 2, it is found that approximately 10% of the total holes of the treated laminated board have insufficient precipitation where plating thicknesses are below the predetermined thickness of 30 .mu.m (the covering ratios are as inferior as 50% or so). An example of thus observed insufficient precipitation 12', 12" is shown in an explanatory view of FIG. 6.